1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet, and more particularly to an image forming apparatus characterized by a color shift detection technique.
2. Description of the Related Art
A conventional electrophotographic color image forming apparatus has a photosensitive drum for carrying toner images and sequentially transfers the toner images in respective different colors onto an intermediate transfer belt or a sheet held on a conveyor belt to thereby form a color image.
However, when the speed of rotation of the photosensitive drum or the intermediate transfer belt changes due to insufficient mechanical accuracy or the like, causing a change in the positional relationship in the transfer position of each color between the photosensitive drum and the intermediate transfer belt, toner images in the respective different colors cannot be perfectly superimposed one upon another. In short, so-called color shift (image shift) occurs.
To solve this problem, an image forming apparatus proposed in Japanese Patent Laid-Open Publication No. S64-6981 employs a method in which a visible image as a position detection mark is formed by each of color-specific image forming units, and the position detection mark transferred onto a traveling member is detected by an associated sensor, whereafter the image forming units are controlled based on detection signals output from the respective sensors so as to correct an image shift.
The above-mentioned proposal makes it possible to correct color shift that occurs after the lapse of a long time period due to a change in the position and size of an image forming unit or the position and size of a component part in an image forming unit, which are caused by changes in the temperature within the color image forming apparatus.
However, it is required to use toner to form the position detection marks, which results in waste of toner.
To overcome the problem, there has been proposed an image forming apparatus in Japanese Patent Laid-Open Publication No. 2001-83856, which detects electrostatic latent image marks written at predetermined intervals on a color-by-color basis on an image carrier having a surface thereof formed of a dielectric material, and controls the speed of rotation of the image carrier based on detection values.
According to this method, since electrostatic latent images are used as position detection marks, it is possible to prevent waste of toner.
However, in the method disclosed in Japanese Patent Laid-Open Publication No. 2001-83856, only color shift in the sub scanning direction is detected, but color shift in the main scanning direction cannot be detected at the same time. Therefore, it is impossible to correct color shift in the main scanning direction.